Audacity is a great sound editor, and with VST plugin effects you will make your final edits and mixes sound even better.
VST plugins are high quality sound processing modules, far better than the ones included in Audacity. There are hundreds of free ones which you can use to enhance your audio. - Noise Reduction, Compressors, Spectral Enhancers- you name it. I'll link to some of the better ones to get you started.

There's guides around for using VirtualDub (and most of its variants) for this task, but VirtualDubMod goes about things slightly differently with its "Stream List" - hence the need for this guide. It also gives you the ability to add the extractions to the Job Control (batch processing), whereas AFAIK the other variants do not.

The following is a list of the various guides and methods posted in this forum for converting stereo to surround sound. It is meant to provide readers with a listing of all the methods posted as well as useful, related information.